A. Field of the Invention
This invention relates to the field of feed forward cancellation systems.
B. Prior Art
Feed forward cancellation systems are known in the art and are described, for example, in U.S. Pat. Nos. 1,686,792; 2,748,201; 3,426,298; and 3,542,301 and in H. Seidel, "A Microwave Feed-Forward Experiment" Bell System Technical Journal, Vol. 50, November, 1971, page 2879, H. Seidel, "A Feed Forward Experiment Applied to an L-4 Carrier System Amplifier", IEEE Trans Commun. Tech, COM-19, No. 3, June, 1971, page 320-325. However, such cancellation systems have been limited to the cancellation of interference generated with respect to a single input signal. Signals are known which comprise two components which are required to be isolated or separated one from the other. Such two component signals may comprise signals which have both horizontally and vertically polarized components.
In a typical ground to space communication system, a horizontally polarized component may be a transmitted signal from the ground station while the vertically polarized component may be a receive signal from a communication satellite. In another example, the horizontally polarized component may originate from one transmitter while the vertically polarized component may originate from another transmitter. Both components may be received on a single line and it is desired to separate these components one from the other.
It has been known to separate two components of a signal by means such as filtering, phase discrimination, amplitude limiting and level detecting. However, depending upon the degree of separation required, such prior methods have left much to be desired since they have resulted in insertion loss, complexity and relatively high costs. Specifically, there are basic limitations on the amount of separation that may be obtained from these prior methods. For example, if the required amount of separation is great enough, then the complexity of the system and the resultant insertion loss increases to a value where the signal level is below the effective receiver threshhold. Thus, where the components are substantially close to each other in frequency or amplitude and are of small relative values, any use of a filter would degrade the signal to noise ratio below the receiver threshhold.
Other signals which have two components may be signals generated using biphasic modulation where one component has a constant relative phase of 0.degree. while the second component is shifted by a constant 180.degree.. Accordingly, the signals are maintained at the same amplitude and differ only in their relative phase characteristics. Another example may be found in which two signals are maintained at the same relative phase but different amplitude and quantized to two specific levels for purposes of two level amplitude modulated transmission. Thus, the signal would comprise one component at a first amplitude and another component at another amplitude.